Method for detecting living human skin

ABSTRACT

A characteristic curve of the impedance of a skin surface is measured as a function of the frequency of an electric AC voltage by applying the voltage to at least one electric conductor which is galvanically or capacitively coupled to the skin surface. The characteristic curve is compared with a reference characteristic curve, which has been generated previously. If the characteristic curve substantially corresponds to the reference characteristic curve, the skin surface is recognized as belonging to living tissue.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of copending InternationalApplication No. PCT/DE99/01974, filed Jul. 1, 1999, which designated theUnited States.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to a method for detecting living human skin.

Modern access authorization systems may, inter alia, use fingerprintsfor identification purposes. An essential precondition for such anidentification is that it must be protected against forgery ormanipulation. In particular, it must be ensured that it is not possibleto obtain access authorization with fake fingers or cut-off fingers. Itis therefore essential also to check, together with the fingerprint,that the person with this fingerprint is alive.

Various methods for an electronic identification of persons aredescribed in International Publication No. WO 95/26013. In addition torecording a fingerprint, these methods can determine whether the personis alive. These methods include recording the pulse frequency orelectrocardiographic signals, measuring the oxygen content of the blood,the skin temperature, the blood pressure or mechanical properties of theskin surface.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a method fordetecting living human skin which is simple and which is suitable, inparticular, for use in conjunction with a fingerprint sensor.

With the foregoing and other objects in view there is provided, inaccordance with the invention, a method for detecting living human skin,the method includes the steps of:

providing a region of a skin surface close to at least one electricconductor by bringing the region of the skin surface in contact with theat least one electric conductor or by providing the region of the skinsurface at a given distance from the at least one electric conductor;

applying an electric potential of a superimposition of frequencies or anelectric AC voltage with a variable frequency to the at least oneelectric conductor;

determining, with a measurement carried out with the electric potentialor the electric AC voltage, an electric impedance as a specific functionof time and/or frequency, by determining a real part and an imaginarypart of the electric impedance or by determining a frequency and anabsolute value of the electric impedance; and

checking whether the specific function corresponds to a referencefunction.

According to another mode of the invention, the electric potential isprovided as a superimposition of frequencies, wherein thesuperimposition results in a voltage pulse or a voltage jump.

According to yet another mode of the invention, the electric potentialis provided by superimposing the frequencies of a limited interval.

According to a further mode of the invention, the region of the skinsurface is provided close to at least two electric conductors bybringing the region of the skin surface in contact with the at least twoelectric conductors or by providing the region of the skin surface at agiven distance from the at least two electric conductors. The at leasttwo electric conductors are electrically insulated from one another andare spaced by a distance of at least 2 mm from one another.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as embodiedin a method for detecting living human skin, it is nevertheless notintended to be limited to the details shown, since various modificationsand structural changes may be made therein without departing from thespirit of the invention and within the scope and range of equivalents ofthe claims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are graphs of ohmic resistances plotted against thelogarithm of a frequency of an applied voltage; and

FIGS. 3 and 4 are graphs of capacitances plotted against the logarithmof a frequency of an applied voltage.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the figures of the drawings, the invention is describedin detail. The method according to the invention utilizes the fact thatliving human skin has a characteristic layer structure. It is of greatimportance for the invention described here that these layers of theskin have clearly different electric conductivities. If these layers arelocated in the electric field of a configuration or assembly ofelectrodes, a resistive/capacitive system with a very characteristicfrequency curve of frequency variation is formed.

FIGS. 1-4 are diagrams in which the ohmic resistance (real part of theimpedance) and the capacitance (proportional to the imaginary part ofthe impedance) are plotted for various conditions against the logarithmof the frequency of the applied voltage. In the basic measurement, anindex finger was applied to (i.e. positioned on) a silicon wafer coveredwith oxide, and the impedance of this configuration was measured.

Families of curves for various finger states or finger conditions areplotted respectively in the graphs of FIGS. 1 and 3. The dashed curve 1refers to a wet finger, the continuous curve 2 to a normal finger, andthe lower dotted curve 3 to a dry finger. The upper dotted curve 4 isbased on the measurement of the tip of a middle finger.

Characteristic curves for two different test persons are illustrated inthe graphs shown in FIGS. 2 and 4. It is easy to see that the samecharacteristic curve shape of these curves results in a fashion that islargely independent of the finger state and of the test person.

The characteristic of the curve shape of the ohmic resistance isparticularly pronounced. This characteristic curve shape can be faked orsimulated only with difficulty when using an artificial finger. In thecase of a cut-off finger the curve shape changes rapidly as a result ofthe skin tissue dying off. In the following it is described how thischaracteristic impedance curve shape can be used to verify theauthenticity and living nature of the finger applied.

To start, a reference characteristic curve is generated in a first step.In this case, it is possible both to measure the frequency coursedirectly, as illustrated in the figures, or else also to use thetemporal course of a measured signal. An example of the latter method isthe application of a voltage jump to the electrodes and the measurementof the temporal course or variation in the charging current. Therespective characteristic curves look entirely different, but areequivalent in principle, since they are correlated with one another viaa Fourier transformation or a convolution. The method used depends onthe respective application. If high requirements are placed on thereliability of the identification, it is possible, for example, toevaluate the real and imaginary parts of the impedance characteristic.In the case of simpler applications, it suffices to use the absolutevalue of the impedance, since this absolute value can be obtained by asimple averaging of the measurement current. The referencecharacteristic curve is preferably generated such that it represents anaverage course for the impedance curve. This can be achieved, forexample, by averaging over a plurality of curves, possibly recordedunder different conditions. The reference characteristic curve ispreferably recorded separately for each person to be identified later.

The selected impedance values in the selected range of the AC voltagefrequency are stored, for example, together with the essentialcharacteristics (minutiae) of the fingerprint. It is then possible, whenchecking the fingerprint, to compare both the fingerprint itself and thecharacteristic curve for the purpose of detecting life with the storedvalues. Since only slight fluctuations are to be established betweendifferent persons (see FIGS. 2 and 4), it is possible, if appropriate,also to use a single reference curve for all persons to be identified.However, when comparing a current or actual characteristic curve withthis stored reference characteristic curve it is then necessary topermit somewhat larger fluctuation ranges i.e. wider tolerance limits.

Instead of using a pure sinusoidal oscillation for the purpose ofmeasuring the frequency dependence, it is also possible to use asuperimposition of frequencies. Such superimpositions, for example pulseshapes (square-wave pulses, sawtooth pulses or the like), are ofteneasier to generate than pure sinusoidal oscillations. The region inwhich the superimposed frequencies are situated can be restricted to aspecific interval width by suitable filtering. The measured values orcharacteristics curves obtained correspond to an averaging of measuredvalues with a sinusoidal excitation. If the interval width of thesuperimposed frequencies is chosen to be sufficiently small, however, itis also possible to use this simplified method to generate asatisfactory characteristic curve, or to record it during the currentmeasurement.

With each identification of a person, the relevant characteristic curveis measured and compared with the reference characteristic curve. Ifthere is a satisfactory correspondence in this case, and theperson-specific measured values (minutiae of the fingerprint) likewisecorrespond to the reference values, the person is considered to havebeen identified and receives the access authorization. Such a comparisonof characteristic curves can be performed in a way known per se byevaluating the difference between the function values. It is possible,for example, to sum or integrate the squares of the difference betweenthe values of the characteristic curves at each frequency, to sum orintegrate the absolute values of these differences, or to determine themaximum of these differences. The accuracy of the comparison can also beraised, if appropriate, by comparing the logarithms or the firstderivatives of the characteristic curves with one another.

The method according to the invention can be carried out in the case ofa fingerprint sensor by using electric conductors in the sensor. Use ismade for this purpose of a sensor in which, in or below a bearingsurface or touch surface for recording a fingerprint, electricconductors are fitted which, upon application of the fingertip, comeinto direct contact with the skin surface (galvanic coupling) or have aspecific distance from the skin surface (capacitive coupling). In thelatter case, a dielectric layer is located, as protective layer orcovering, between the conductor and the bearing surface for the finger,for example.

For measuring purposes, it is possible to use a single conductor or twoconductors electrically insulated from one another. If only oneconductor is used, the finger applied acts as a connection to thegrounding potential. In the case of the use of two electric conductors,the conductors are preferably provided at a distance which is greaterthan the thickness of the epidermis. The method can therefore be carriedout using conductors which are at a distance of at least 2 mm from oneanother. It is sufficient if the conductors are two metal plates withapproximate dimensions of 10 mm². Depending on the desired measuringresolution, it is also possible to employ substantially smallerdimensions. The impedance can be measured in a way known per se. It ismerely necessary to ensure that the selected measuring method supplies aresult which is sufficiently accurate for the purpose. If the method isused in the case of a fingerprint sensor, the conductor or theconductors for detecting life is or are preferably provided at the edgeof the bearing surface for the fingertip. However, since, as a rule, thesensor itself is constructed from electrically conductive sensorelements, individual ones of these sensor elements can also be used tocarry out the method described. The method can therefore also be carriedout in principle with conventional sensors by using suitable electronicdevices.

I claim:
 1. A method for detecting life from human skin, the methodwhich comprises: one of contacting a region of a skin surface with atleast one electric conductor and applying the region of the skin surfaceto a bearing surface or touch surface at a distance from the at leastone electric conductor; applying an electric potential of one of asuperimposition of frequencies having one of a voltage pulse and avoltage jump and an electric AC voltage with a variable frequency variedover a selected range to the at least one electric conductor;determining, with a measurement carried out with the electric potential,a real part and an imaginary part of or an absolute value of an electricimpedance as a function of one of time and frequency; and checking ifthe determined function corresponds to a reference function.
 2. Themethod according to claim 1, which comprises providing the electricpotential by superimposing the frequencies from a limited interval. 3.The method according to claim 1, wherein the at least one electricconductor are at least two electric conductors, and which comprises: oneof contacting the region of the skin surface with the at least twoelectric conductors and bringing the region of the skin surface to thegiven distance from the at least two electric conductors; andelectrically insulating the at least two electric conductors from oneanother and spacing the at least two electric conductors by at least 2mm from one another.